The present invention relates generally to magnetic recording, and more particularly to apparatus and methods of obtaining the position of a magneto-resistive head relative to a magnetic track. Due to the advancement and proliferation of technology, there is a constant need to store more information in a storage medium. A common form of the storage medium uses magnetic transitions on defined tracks of a surface of the medium. The transitions are written and read back by a magnetic head. In order to have more information on the tracks, there is a constant push to increase the track density by scaling down the width of the tracks. However, the higher the track density, the more difficult it is to accurately position the magnetic head over a track. During both the writing and reading process, the position of the head must be controlled to guarantee adequate signal quality for the required data integrity in a practical product environment.
The position information in the form of magnetically encoded marks have been traditionally written on the medium to carefully control the position of the head with respect to the track. Signals from these marks are read by the head and manipulated to provide position measurements. These position measurements are then used by a position controller to more accurately position the head with respect to the track.
Two methods are commonly used to encode the position information. They are the dedicated-servo method and the embedded-servo method. The dedicated-servo method is typically used for a disk drive with many surfaces to store information. One surface, known as the servo surface, is dedicated to position information only, while the remaining surfaces, known as the data surfaces, contain primarily user data. Each surface has a magnetic head to read and write data onto the surface, and all the heads are mechanically tied together. While one head, the servo head, follows the position data from the servo surface, the remaining heads, the data heads, are slaved to the servo head. As track densities continue to increase, a relative shift in position between either the servo head and the data heads, or the servo surface and the data surfaces may create unacceptable amount of position variations. These variations may be reduced by known alignment schemes. But even with such schemes, it is generally believed that the dedicated-servo method may not be extendable to storage media with high track densities.
In the embedded-servo method typically used in disk and some tape storage products, position information is encoded on every track, and is interspersed with user data. This scheme allows each magnetic head to follow every track on its own surface by reading position information directly from this surface. However, the position information tends to consume track space, which could otherwise be used for user data.
In search of alternative schemes to enable simultaneous reading of user data and relative head position, researchers have come up with different methods. One method center-taps a magneto-resistive (MR) head to split the MR head. This essentially results in two elements, oriented side-by-side. A difference signal from these two elements provides position information of the head relative to the track while the sum of these same signals simultaneously provides the desired data information. Due to lithographic resolution and tolerance limits, it is quite difficult to reduce the width of the center tap for high density tracks. As the width of the track reduces in dimension, the ratio of the center tap width to the head width becomes larger. This leads to a significant portion of the head width being consumed by the center tap, and a reduction in the data signal quality.
Another prior art method uses two electrically isolated MR elements to provide position information and user data information simultaneously. In this case, the two elements are again aligned side-by-side. But, this time, the alignment is done mechanically via lithography, with the two elements right adjacent to each other. As the width of the track decreases to increase the track density, such careful alignment becomes harder and harder to achieve.
It is apparent from the foregoing that there is a need for a magnetic head that is applicable to storage media with high track density, and does not unreasonably consume track space on the surfaces of the storage media otherwise available for storing user data.